(1) Field of the Invention
The present invention relates to an engine controller for a vehicle equipped with an automatic centrifugal clutch, such as a snowmobile, ATV (all-terrain vehicle), motorcycle, etc., including an idle speed control (abbreviated hereinbelow as ISC) device disposed in the engine intake system for adjusting the amount of intake air by a throttle valve.
(2) Description of the Prior Art
Concerning engine intake systems for regulating the amount of intake air based on control of the opening degree at the throttle valve of an engine mounted on vehicles, such as snowmobiles, having an automatic centrifugal clutch, there are those that adopt ISC devices for controlling the idle speed by adjusting the amount of bypass air.
In a vehicle including such an ISC device, an ECM (electronic control module) is provided for the ISC device, so as to control the amount of intake air at the time of the throttle valve being fully closed, for each of the following modes (as to the prior art relating to the ISC device, refer to Japanese Utility Model Application Laid-open Sho 64 No. 34441 (FIG. 1 and the full text of the specification)).
In ISC, in startup mode, control at a start of the engine is performed by adjusting the amount of bypass air to the controlled amount for start in accordance with the cooling water temperature plus corrections for altitude and other factors.
In the complete explosion mode other than the startup mode, the basic control amount is corrected in accordance with the cooling water temperature and at the same time the idle speed is controlled by regulating the amount of bypass air.
In vehicles having an automatic centrifugal clutch, e.g., snowmobiles, a typical configuration which has such a configuration that the clutch becomes engaged to transfer the driving force from the engine to the drive shaft when the engine speed reaches about 2800 rpm is adopted.
Therefore, if the engine speed exceeds the clutch engagement rotational frequency, there is a possibility of the vehicle starting to run regardless of the rider's intention.
To avoid this, in the ISC device, the amount of bypass air at startup mode is controlled by determining the controlled value of the amount of bypass air of ECM so that the engine speed will be lower than the clutch engagement rotational frequency.
However, startup of an engine with ISC at very low temperatures or use of it at a high-altitude site where air density is low needs to be considered, a configuration having a large bypass air capacity should be selected. In this case, if the ISC device breaks down and stops with its full-open side, the aforementioned controlled value is invalidated at startup, so that an excessive amount of bypass air is supplied to increase the engine speed, whereby there arises a risk that the automatic centrifugal clutch becomes engaged in error, causing the vehicle to start running.
In the above ISC device, there is a possibility that the engine speed might increase due to breakdown of ISC, not only at startup but also when the engine is retarded with the throttle cut off. This should also be prevented. That is, occurrences of malfunction are not limited to the state where the travelling speed is zero (the vehicle being stopped).
In this case, however, if fuel cutoff (or ignition cutoff) control is performed when the engine speed becomes exceeding a set engine speed (this configuration has not been published), it is possible to prevent the vehicle from starting to travel unexpectedly. However, when the vehicle is retarded by completely closing the throttle valve into its off-throttle state, fuel supply is cut off at engine speeds greater than the set engine speed no matter what speed the vehicle travels at. Particularly, excess engine brake acts on the vehicle when it is retarded from a high travelling speed, causing significant degradation of rider's sensation during traveling.